Method and injection molding machine for the production of elastomer and duromer molded items

ABSTRACT

A method for the production of elastomer and duromer molded items in an injection molding process using an injection molding machine with a respectively separately heatable filling unit, injection unit and cross-linking tool. The filling unit is heated to a temperature which is non-critical with regard to the cross-linking reaction of the plastic to be treated. The injection unit is heated to a temperature which is slightly below the cross-linking temperature of the molding material used.

FIELD OF THE INVENTION

The present invention relates to a method for producing elastomer andduromer molded parts. More particularly, the present invention relatesto a method for producing elastomer and duromer molded parts usinginjection molding methods and injection molding machines each having aseparately heatable filling unit, injection unit and cross-linking tool.The filling unit and the injection unit being heated to a temperaturethat is not critical for the cross-linking reaction of the plastic to betreated. At the same time, an injection molding machine for carrying outthe method is described.

BACKGROUND INFORMATION

Injection molding methods and the injection molding machines requiredfor them are widespread. In order to achieve good utilization of themachines, it is attempted to make vulcanization times as short aspossible. The vulcanization time of component parts having a wallthickness of 1 cm is several minutes.

In order to decrease the vulcanization time, an injection moldingmachine has become known from U.S. Pat. No. 4,370,150, in which aperiodically operating heating device is put in between the filling unitand the cross-linking tool, in which a part of the elastomer pressed inby the filling unit is heated to a temperature which just aboutcorresponds to the cross-linking temperature in the cross-linking tool.The cross-linking process is repeated for each injection procedure.Here, a kind of pre-cross-linking of the elastomer takes place, so thatthe final cross-linking in the cross-linking tool is very much shortenedin time. Such a device, however, has the disadvantage that, in theheating zone, cross-linking may occur, which impedes the injectionprocedure. In the usual injection molding machines for producingelastomer and duromer molded parts, the molding compound in the fillingunit is plastified at a temperature that is non-critical to thecross-linking reaction, mostly between 70° and 90° C. Only afterinjection into the cross-linking tool, which is preheated to thecross-linking temperature, does the cross-linking reaction begin.Cross-linking reaches its maximum speed when the molding compound hasreached the cross-linking temperature. Because of the low heatconductivity of the cross-linking molding compounds, the cycle time ofthe individual injection procedure is essentially determined by thecross-linking time. In this context, the heat transportation from thewall of the tool to the inner part of the molded part, so as to reach amaximum cross-linking speed, is the deciding factor.

SUMMARY OF THE INVENTION

The present invention is based on the object of further developing theinjection molding method and the appertaining injection molding machine,in order to increase their productivity. The cross-linking time in thecross-linking tool of especially thick-walled molded parts is to beclearly reduced. In addition, the molded part should have a more uniformcross-linking structure.

According to an exemplary embodiment of the present invention, theinjection unit itself is heated to a temperature that is slightly belowthe cross-linking temperature. In experiments it was shown that, by thismeasure, the vulcanization time was reduced quite considerably. Theretention time of the elastomer in the injection unit is preferablyreduced to a minimum. Additional parts on the injection molding machine,such as the special heating path named above, are not required.

The injection molding machine for carrying out the method is provided ina known manner with a filling unit, an injection unit and across-linking tool, which are separately heatable. In order to achieve aretention time of the molding compound that is as short as possibleafter leaving the filling unit, the injection unit is furnished with anannular cylinder having an annular piston. The heating of the annularcylinder and of the core of the cylinder may be done by appropriatelydesigned heating elements. These heating elements may be regulatedsimultaneously and synchronously. The advantage of the preheating of themolding compound in the injection unit is that, directly after thecomplete filling of the cavity in the cross-linking tool, cross-linkingbegins at maximum speed. In spite of the poor heat conductivityproperties of the molding compounds to be cross-linked, an essentiallyhomogeneous temperature distribution is achieved over the entire crosssection.

This, in turn, causes a largely uniform cross-linking structure over theentire component part thickness, since the cross-linking reactions setsin in all regions of the material that has been preheated close to thecross-linking temperature inside the cavity, approximatelysimultaneously at maximum speed. This enables one to achieve a highquality of the elastomer molded parts, at shortened cycle time, in theproduction of the parts. The cylinder chamber is preferably heated to atleast 90% of the cross-linking temperature. Very favorable results areobtained if the heating is carried out up to 95 to 98% of thecross-linking temperature. As a rule, the volume of the molding compoundcontained in the cylinder chamber and heated close to cross-linkingtemperature is selected so that it corresponds to the volume of the toolcavity. A complete exchange of the molding compound in the cylinderchamber is thereby ensured. The piston ring space is able to be closedoff on its underside by a slider. The volume for individual injectionprocedures may be varied by the appropriate development of the pistonstroke.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic illustration of an example embodiment of theinjection molding machine of the present invention during filling.

FIG. 2 is a schematic illustration of an example embodiment of theinjection molding machine of the present invention after filling.

FIG. 3 is a schematic illustration of an example embodiment of theinjection molding machine of the present invention prepared for theinjection procedure.

FIG. 4 is a schematic illustration of an example embodiment of theinjection molding machine of the present invention after carrying outthe injection procedure.

DETAILED DESCRIPTION

In FIG. 1, an injection molding machine 1 is shown schematically, whichis made up of filling unit 2, injection unit 3 and cross-linking tool 4.Injection molding machine 1 is shown during the filling procedure.Molding compound 6 introduced into feed region 5 is conveyed by wormgear7 and the piston of filling unit 2 into ring space 8 of injection unit3. Annular piston 9 moves upwards in this context, as shown by arrow 10.Pressure piston 11 is affixed at the upper end of annular piston 9, andit is needed for the injection procedure. Wormgear 7 is driven by motor12, using a gear 13 placed in between. Wormgear housing 14 is furnishedwith separate heater to correct temperature 15. Ring cylinder 16 isequipped with heater to correct temperature 17. Preferably, cylindercore 18 is also furnished with heater to correct temperature 19.Cross-linking tool 4, which is located below injection unit 3, isprovided with its own heater 20. The volumes of tool cavity 21 and ofpiston ring 8 are matched to each other in such a way that a filling ofpiston ring space 8 corresponds to a filling of tool cavity 21. A slider22 at the underside of annular piston space 8 is used to close pistonring space 8 after filling.

The reference numerals in the following figures correspond to the partsdescribed in FIG. 1.

In FIG. 2 the filling of piston ring space 8 is closed off. Wormgear 7is standing still. Slider 22 has closed off annular piston space 8. Asin FIG. 1, the temperature at the filling unit is ca 80° C., thetemperature at injection unit 3 is ca 165° C., and the temperature atcross-linking tool is ca 180° C.

In FIG. 3, injection unit 3 is connected to cross-linking unit 4, andthe slider is open. Filling unit 2 is still in a position of rest.Injection unit 3 begins the injection procedure, which is indicated byarrow 30. The temperatures set in all three machine parts aremaintained.

FIG. 4 shows the end of the injection procedure. Annular piston 9 hasreached the stop at its lower end, and molding compound 6 has beensqueezed out of piston ring space 8 into cavity 21 of cross-linking tool4. This ends one cycle of the injection procedure, and the procedure isable to begin anew when the parts are assembled as in FIG. 1.

1. A method for producing elastomer molded parts and duromer moldedparts using an injection molding machine comprising a separatelyheatable filling unit, injection unit and cross-linking tool, theinjection unit having an annular cylinder, comprising the steps of:introducing molding compound into the injection unit; heating thefilling unit to a temperature that is non-critical with regard to thecross-linking reaction of a plastic to be treated; heating the core ofthe annular cylinder of the injection unit to a temperature that isslightly below the cross-linking temperature of the molding compound;and closing off the annular cylinder with a slider.
 2. The method asrecited in claim 1, wherein the injection unit is heated such that aretention time of the molding compound in the injection unit isminimized.
 3. A method for producing elastomer molded parts and duromermolded parts using an injection molding machine comprising a separatelyheatable filling unit, injection unit and cross-linking tool, theinjection unit having an annular cylinder, comprising the steps of:introducing molding compound into the injection unit; heating thefilling unit to a temperature that is non-critical with regard to thecross-linking reaction of a plastic to be treated; heating the annularcylinder of the injection unit to a temperature that is slightly belowthe cross-linking temperature of the molding compound; and closing offthe annular cylinder with a slider.
 4. An injection molding machine forcross-linking molding compounds comprising: a filling unit; an injectionunit; and a cross-linking tool, wherein the filling unit, the injectionunit and the cross-linking tool are each configured to be separatelybrought to predetermined temperatures, wherein the injection unit has anannular cylinder having an annular piston, and wherein the core of theannular cylinder is provided with a heating element, and wherein theannular cylinder is configured to be closed off on its underside by aslider.
 5. The injection molding machine as recited in claim 4, whereinthe injection unit is heated to a temperature that is slightly below thecross-linking temperature.
 6. The injection molding machine as recitedin claim 4, wherein the outer portion of the annular cylinder isprovided with a heating element.
 7. The injection molding machine asrecited in claim 6, wherein the heating elements of the core and theouter portion of the annular cylinder are configured to besimultaneously and synchronously regulated.
 8. The injection moldingmachine as recited in claim 4, wherein a volume of a cross-linkingmolding compound is contained in an annular space below the annularpiston in the annular cylinder and is heated to nearly the cross-linkingtemperature, said volume corresponds to the volume of the tool cavity.